It was recently demonstrated that in a liquid scintillation spectrometer hard p-emitters (""P, 24Na, lo6Rh, 144Pr, Q"Y, 40K, *04T1, s"Sr, 13'Cs, 42K) can be measured in aqueous solutions without the addition of scintillator (l-3). This is due to the fact that fast fl-particles in water give rise to cerenkov radiation, which can be detected by the photomultipliers of a liquid scintillation counter.
When a charged particle is moving through a transparent medium at a speed greater than that of light within the same medium, an electromagnetic shock wave is produced. Given the velocity of the particle w, the velocity of light c, and the refractory index of the solvent n, CerenkoL radiation arises when (v/c)n > 1 (4). This requires that the energy of the p-particles must be larger than 0.265 MeV.
From the P-spectrum of 3?P it can be calculated that 85% of the p-particles have energies above 0.265 MeV (5). Therefore, it should be possible to detect this isotope in aqueous solution by measuring the cerenkov radiation produced. The aim of this report is to describe the utilization of cerenkov radiation for the measurement of 3"P.
A liquid scintillation counter (Tri-Carb, model 3314, Packard) was used for the measurements. 32P was purchased at The Radiochemical Centre, Amersham, England. Glass vials were used throughout. The high voltage of the apparatus was adjusted to give a maximum efficiency (42.2%) for the counting of a tritium standard (Packard No G U71tritiated toluene dissolved in toluene with 0.4% 2,5-diphenyloxazole (PPO) and 0.03:/o 1,4-bis-2-(4-methyl-5-phenyloxazolyl) benzene (dimethyl-POPOP))
at a gain of 16% with a window setting of 50-1000 (Fig. 1). 32P dissolved in aqueous solution of 10 mM KH,PO, was counted in the same window; 50 was chosen as the lower limit in order to reduce the background. For a volume of 10 ml, the efficiency of 32P counting was shown to attain its maximum at a gain of 16% (Fig. 1).
When the high voltage was increased, the efficiency could be augmented.